1. Field of the Invention
This invention relates to an improvement of a process for producing a three layer co-extrusion biaxially polypropylene (hereinafter as BOPP) synthetic paper a thickness of from 25xcx9c250 xcexcm. More particularly, the invention relates to a process for producing a three layer BOPP synthetic paper by means of a three layer co-extrusion process wherein two different polypropylene (PP) resin compositions are first separately extruded by one primary single screw extruder with venting device and two secondary single screw extruders with venting device and then are co-flowed by the same T-die to form a three layer coating sheet, and through cooling biaxial orientation, corona treatment and winding to form a three layer co-extrusion synthetic paper of thickness 25xcx9c250 xcexcm. The three layer structure of said three layer co-extrusion synthetic paper of thickness 25xcx9c250 xcexcm can be made into three kinds, such as: paper sheet layer/foamed intermediate layer/paper sheet layer with double side paper sheet layer, paper sheet layer/foamed intermediate layer/resin layer with single side paper sheet layer and resin layer/foamed intermediate layer/resin with double side high gloss resin layer, to use as a coating substrate for coating grade synthetic paper.
2. Description of the Related Art
Nowadays the three layer co-extrusion BOPP synthetic paper differs with the polyolefin synthetic paper mainly comprising the basic substrate layer in the intermediate layer, whereon the back surface is laminated with the uniaxially oriented polypropylene (UOPP) containing inorganic fine powder as the paper surface layer(such as disclosed in the Japanese Patent Publication No. 40794/71, Japanese Laid-open Patent Publication No. 41339,81, Japanese Laid-open Patent Publication No. 118437/81 and Japanese Laid-open Patent Publication No, 87225/91, and Taiwan Patent Publication No 72218). The high strength of the paper sheet of the three layer co-extrusion BOPP synthetic paper obtained by these patent processes the problem caused by paper powder formation during printing process, and owing to the fact that the thermal dimensional stability of the paper sheet layer is good enough and cost is very low, so these synthetic papers can be sold widely sold.
However it must be improved in some aspects, such as hiding power, heat resistance, total thickness and cost of BOPP synthetic paper to replace natural pulp in wide applications. These applications can be industrialized package usage such as packaging paper, kraft paper and releasing paper etc.
Although the above mentioned patent publications have been granted, there are still some problems to make use of BOPP synthetic paper in wide applications.
However, BOPP synthetic paper can be used in various applications if the hiding power, heat resistance, total thickness of BOPP synthetic paper product could be enhanced. This, however requires improvement from the viewpoint of, for example depending on the total amount of inorganic powder added.
Upon consideration of the dispersion problem of inorganic powder used in the single screw extruder, only masterbatch ready made by dispersing the inorganic powder with resin can be used. The masterbatch mixes with the polypropylene resin in the inlet of the extruder, then enters into the extruder to melt and extrude. To obtain the dispersion of the inorganic powder in the production of the masterbatch, it is usual to use a dispersing agent such as coupling agent, calcium stearate to enhance surface treatment of the inorganic powder, to obtain much better productivity (to avoid film break in the drawing zone)and product appearance, evenness of BOPP synthetic paper. But the masterbatch containing the inorganic powder with dispersing agent incurred and moisture adsorbed may volatilize its dispersing agent and moisture into gas under high temperature kneading by the screw in the extruder. This will cause voids to be formed in the sheet during cooling after extrusion. The existing voids also form gas bubble in the product of latter zone drawing. It is necessary to control the formulation condition and processing condition.
It is a primary object of the present invention to provide an improvement of a process for producing a three layer co-extrusion biaxially polypropylene (hereinafter as BOPP) synthetic paper of thickness 25xcx9c250 xcexcm.
It is another object of the present invention to provide a process for producing a three layer BOPP synthetic paper by means of three layer co-extrusion wherein two different PP resin compositions are separately extruded by one primary single screw extruder with a venting device and two secondary single screw extruders with a venting device first, and then are co-flowed by the same T-die to form a three layer coating sheet, and through cooling, biaxial orientation, corona treatment and winding to form a three layer co-extrusion synthetic paper of thickness 25xcx9c250 xcexcm.
It is a further object of the present invention to provide three kinds of three layer co-extrusion synthetic paper of thickness 25xcx9c250 xcexcm, such as: paper sheet layer/foamed intermediate layer/paper sheet layer with double side paper sheet layer, paper sheet layer/foamed intermediate layer/resin layer with single side paper sheet layer and resin layer/foamed intermediate layer/resin with double side high gloss resin layer, to use as coating substrate for coating grade synthetic paper.
The inorganic powder may be added as needed to meet the need of the product application and quality, the applicant adopt the following two ways to reach this aim, namely to vent away the dispersing agent and moisture in the masterbatch to avoid the abnormality occurs during processing.
(1) First, each ingredients used in the extruder should be pelletized into masterbatch by pelletizer. The pelletizer adopts natural venting means to expel the remaining dispersing agent or moisture, and volatile ingredient. Then the masterbatch prepared is transferred into extruder for succeeding step.
(2) The three single screw extruders used in this process should be changed in screw structure (to make reducing zone) and install new venting device to expel the remained dispersing agent and moisture or volatilable gas. In spite of adding any amount of inorganic powder, there is no hollow void may be found in the sheet obtained after cooling.
The above described improvement ways can reach the aim of obtaining quality elevation in hiding power, heat resistance, total thickness of product and cost reduction. It is preferably to adopt the (2) means under the consideration of production cost, raw materials used and storage management.
The BOPP synthetic paper obtained by the venting process of single screw extruder which was manufactured without limitation in adding inorganic powder has much more advantages in increasing quality and cost reduction. This technical feature has not been disclosed in the prior art. The advantageous effects of this invention will be described as follows:
(1) Increasing Hiding Power
The hiding power of BOPP synthetic paper obtained by the biaxial drawing process depends on the amount of masterbatch and quantity inorganic powder. If inorganic powder can be added in large amounts, the foaming formation of product in the drawing zone is satisfactory, and hiding power from light is also good. Consequently, high hiding power can be reached without adding masterbatch or with reducing masterbatch amount used.
(2) Increasing Heat Resistance
The thermal barrier of inorganic powder is better than that of polypropylene resin, preferably used inorganic powder are calcium silicate and talc. The addition of larger amounts of inorganic powder will increase heat resistance and increase high foam formation.
(3) Increasing Total Thickness
The amount of inorganic powder added and the specific gravity of product have good correlation, and the specific gravity of product is the dominating factor of product thickness determination. If the specific gravity of product can be effectively decreased, the total thickness of product will be increased.
(4) Cost Reduction
If the amount of inorganic powder can be increased, it can reduce the cost in compounding formulation and the specific gravity of product. Consequently, the cost of synthetic paper also can be reduced.
The above described advantageous effects of the BOPP synthetic paper obtained by this invention using single screw extruders can be reached to increase the product quality and to reduce the cost, moreover the BOPP synthetic paper can be widely applied in various industrial package uses.
The BOPP synthetic paper of this invention having a three structure, which is constructed as paper sheet layer or resin layer/foamed intermediate layer/paper sheet layer or resin layer. The polypropylene (PP) resin composition used for foamed intermediate layer is extruded by a single screw primary extruder with a venting device. While the PP resin composition used for the paper sheet layer or resin layer is extruded or are extruded by two single screw secondary extruders with venting device. The above mentioned extrudates are co-flowed by the same T-die to co-extrude and form a three layer coating sheet, and through cooling, biaxial orientation, corona treatment, and winding to form a layer co-extrusion BOPP synthetic paper of double side, single side paper sheet layer and double side resin layer with high gloss.
The PP resin composition used for foamed intermediate layer is extruded by one single screw primary extruder with venting device. The PP resin composition which was composed of the polypropylene (PP) resin of 30xcx9c96% weight having high crystallinity of isotacticity above 97%, antistatic agent of 1xcx9c5% weight, calcium carbonate masterbatch 3xcx9c50% weight, and titanium oxide masterbatch 0xcx9c15% weight is uniformly blended into front side of the single screw extruder, and the volatile gas incurred in the PP resin composition can be expelled by the venting device. The PP resin composition was blended to melt toward the intermediate runner of the T-die. While the PP resin composition used for the paper sheet layer or resin layer are extruded by two single screw secondary extruders with venting devices. The PP resin composition which was composed of the polypropylene (PP) resin of 40xcx9c100% weight, polyethylene resin of 0xcx9c20% weight, antistatic agent of 0xcx9c5% weight, calcium carbonate masterbatch of 0xcx9c35% weight, titanium oxide masterbatch of 0xcx9c20% weight, antiblocking agent of 0xcx9c5% weight and ultraviolet ray absorber of 0xcx9c5% weight is fed into inlet in the front side of the two single screw secondary extruders with venting devices and uniformly blended, and the volatile gas incurred in the PP resin composition can be expelled by venting device. The PP resin composition was blended to melt toward the two outer runner of the T-die. The three extrudates extrude from the three single screw extruders were co-flowed through the same T-die in the way of three layer co-extrusion to form the coating sheet of paper sheet layer or resin layer/foamed intermediate layer/paper sheet layer or resin layer. Then, the said coating sheet was subjecting to biaxial orientation, corona treatment, winding to form a three layer co-extrusion BOPP synthetic paper of thickness 25xcx9c250 xcexcm with double side, single side paper layer or double side resin layer with high gloss.
For the understanding of technical content of this invention, here is the detailed description of the aspects including polypropylene resin compositions, process (including extrusion, three layer co-extrusion, cooling, biaxial orientation, corona treatment, winding)as follows.
PP resin used in this invention which having high crystallinity as the essential ingredient to be suitable not only for cultural paper, but also for industrial package paper. The PP resin used in this invention are homopolymer with high crystallinity of melt flow index 0.5xcx9c12 (230xc2x0 C./2.16 kg, ASTM DI238) and most of them are isotactic. Different MFI and molecular weight distribution of PP resin can be chosen to match the drawing ratio in controlling the mechanical strength and thickness evenness. To attain the imitation paper effect of different gloss by using paper sheet layer, it is preferable to choose PE (polyethylene) of MFI 0.1xcx9c14.
The inorganic powder used in this invention mainly plays the function for significantly decreasing the specific gravity, elevating the hiding power, heat resistance and total thickness, and thus provides the paper sheet layer with excellent pen-writing and printability that is superior to conventional synthetic paper. The inorganic powder can be one or more than two selected from the group of calcium carbonate, diatomite, kaolin clay, calcium oxide, barium sulfate and titanium oxide, and the particle size of inorganic powder ranges from 0.1 to 10 xcexcm, the amount used depends on the physical properties required. The inorganic powder used in this invention should be made into masterbatch before processing, kind and concentration of dispersing agent used in the masterbatch also depends on the kind of inorganic powder and product application. Titanium oxide masterbatch also can be used to provide the function for adjusting the opaqueness, whiteness of synthetic paper and enhancing the effect of ultraviolet resistance, this also depends on product application.
The antistatic agent used in this invention could be the same as those used in the BOPP, in which tertiary amine, amide preferably used can eliminate the static effect caused in the coating process.
In order to prevent the mutual adhesion between the coating layers from ocurring while coating and winding process, the antiblocking agent is required, and generally it may be such as silica, clay, polymethyl methacrylate(PMMA), and glass bead.
The specific gravity of BOPP synthetic paper obtained by this invention falls into the range of 0.5xcx9c0.89, this can be adjusted in formulation of PP composition according to the product application and cost. It is intended to let the consumer to use wider coverage in the same unit weight of product.
Three layer structure of the BOPP synthetic paper obtained by this invention is paper sheet layer or resin layer/foamed intermediate layer/paper sheet layer or resin layer as described above, the physical properties, productivity depend on the formulation, equipment, operation condition required. Be able to add large amount of inorganic powder to enhance the production of BOPP synthetic paper is the characteristic feature of this invention, and this has not been reached by prior art, especially in biaxial orientation drawing process. BOPP synthetic paper of thickness 25xcx9c250 xcexcm in single, double side matted paper sheet layer and two side high gloss paper sheet layer can be obtained by the process shown in the accompanying drawing. The device, steps used can be described as follows.
To achieve the above objects, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.